Optical system for a multidetector array spectrograph

ABSTRACT

An optical system for a multidetector array spectrophotometer which includes multiple light sources for emitting light of selected wavelength ranges and means for selectively transmitting the selected wavelength ranges of light to respective slits of a multi-slit spectrogrpah for multiple wavelength range detection. The spectrograph has two or more slits which direct the selected wavelength ranges of the light spectra to fall upon a dispersive and focusing system which collects light from each slit, disperses the light by wavelength and refocuses the light at the positions of a single set of detectors.

BACKGROUND OF THE INVENTION

This application is a continuation-in-part of application Ser. No.191,262, filed May 6, 1988, now U.S. Pat. No. 4,875,773, entitled "NEWOPTICAL SYSTEM FOR A MULTIDETECTOR ARRAY SPECTROGRAPH".

FIELD OF THE INVENTION

This invention relates to an optical system for a multidetector arrayspectrograph to be used as a component of a spectrophotometer capable ofmeasuring the absorbance of light by a sample as a function of thewavelength of light passing through the sample. This invention furtherrelates to an optical system for a multidetector array spectrograph tobe used as a component of a radiometer or emission spectrophotometercapable of measuring light energy emitted by a sample as a function ofthe wavelength of light. This invention further relates to an opticalsystem for a multidetector array spectrograph to be used as a componentof a reflectance spectrophotometer capable of measuring the reflectanceof light from a sample as a function of light wavelength. This inventionfurther relates to an optical system for a multidetector arrayspectrograph to be used as a component of an instrument capable ofmeasuring the fluorescence or phosphorescence of a sample. Thisinvention further relates to an optical system for a multidetector arrayspectrograph to be used as a component of an instrument capable ofmeasuring the emission spectra or radiated energy of a sample as afunction of wavelength.

DESCRIPTION OF THE PRIOR ART

For some years it has been known that meaningful laboratory analysis canbe performed using instruments which measure the light absorbed by asample, or reflected from a sample, or emitted from a sample as afunction of wavelength. Spectrophotometers which measure the absorptioncharacteristics of materials were produced by Bausch & Lomb in 1953. Theabsorption may be indicative of the presence of an impurity in a liquidunder test, of solute in a solvent, of the color of the liquid, of thepresence of solid matter suspended in the liquid, or the like. Numerousinstruments for such applications are know. The art has well documentedthe wavelengths of light which are absorbed by various materials so thatthe absorption of light of a specific wavelength is indicative of thepresence of a particular material in the sample under test. If theamount of incident light and transmitted light are compared, anindication of the amount of the absorptive material may be derived.

For these reasons, it is often desirable to make measurements of theamount of light that a sample absorbs as a function of the wavelength oflight. It has also proven desirable to measure light absorption by asample for selected ranges of wavelengths of light. Prior approaches tomeasuring light absorbed by a sample as a function of wavelength havetypically utilized one or two very well known techniques.

One prior art technique is to generate white light and direct thegenerated white light into a monochromator. The monochromator receiveswhite light emitted by the source and produces a monochromatic light ofa selected wavelength by allowing only the small band of selectedwavelengths to emerge from the exit slit of the monochromator. The lightemerging from the monochromator travels through a sample underinvestigation. Typically, a portion of the light entering the samplewould be absorbed by the sample itself. The remaining monochromaticlight passing through the sample is measured by a single detector placedon the other side of the sample cell. In order to measurecharacteristics of the sample to absorb light over a range ofwavelengths, the above experiment would be repeated for each wavelengthin the selected range of wavelengths by adjusting the grating in themonochromator such that a next wavelength in the selected range would beemitted. Measurements would be repeated for each wavelength within theselected range. Such a procedure would successfully permit anexamination of sample absorbency of light over a range of wavelengths.One such spectrophotometer has been marketed by the Milton Roy Companyunder the name " SPECTRONIC™ 2000". The primary disadvantage to deviceswhich use such an approach is that when measurements of sampleabsorbency for a relatively large wavelength range is desired, therepeated adjustments to the monochromator would result in a relativelylengthy time to acquire data over the desired wavelength range.

The second technique is to measure sample absorption over wavelengthranges is to generate and transmit white light directly through thesample under investigation. Light passing through the sample is directedto a spectrograph where an array of photodetectors would simultaneouslyread the absorbency of the sample at a number of wavelengthssimultaneously. In order to get high resolution of sample absorptionreadings over the large wavelength range typically present in such amethod, various approaches have been taken.

One approach for measuring over a wide range of wavelengths is to use adetector array with a very large number of detecting elements inconjunction with a fixed grating and entrance slit. Unfortunately, suchdetector arrays tend to be very expensive, and therefore undesirable formany uses. Also, as detector arrays are planar, focus of the spectrumover the long array length is inherently poor.

Another approach includes the use of an array where the range ofwavelengths studied could be changed by rotating the grating to a newlocation. This approach has proven undesirable as the mechanical systemwhich positions the grating tends to adversely affect the accuracy ofthe device. Thus, to get repeatable results, the mechanical system whichutilizes the reduced size detector array must be able to locate andposition the grating with a high degree of precision. One suchspectrophotometer has been marketed by Perkin-Elmer.

Yet another approach was to direct light from the entrance slit onto twoor more gratings. Light from the gratings would be directed onto acorresponding sensor array. Such a system requires the proper alignmentof numerous mechanical and optical components and proved too expensivefor many spectrophotometric applications. One such spectrophotometer hasbeen marketed by Hewlett Packard as their model 8450A spectrophotometer.

All of the above described techniques involve the use of a spectrographwhere a single slit or aperture of various shapes is provided for lightto pass through in combination with various dispersive and focusingsystems for collecting light from the slit, dispersing the light bywavelength and refocusing at detectors. The limitation of the singleslit designs described above is that resolution is limited by the numberof detectors. It has proven difficult and expensive to obtain theelectronics for a large number of detectors and to form an accurate highresolution image over the larger range of positions required for a largenumber of detectors.

Furthermore, if the detectors are small and close together, it maybecome optically difficult to disperse the wavelengths accuratelywithout mixing in light from incorrect wavelengths. This is one sourceof stray light which is a type of error in dispersive instrumentation.Since gratings disperse light by means of interference effects, gratingsimage some light of wavelength lambda, one half lambda, one quarterlambda, etc., in the same place. When an instrument is designed suchthat the longest wavelength to be analyzed is more than twice theshortest wavelength to be analyzed and a grating is used as thedispersive element, then order filtering must be used to suppress thelight from the half wavelength values which would normally reach thedetector. For example, in a system designed to detect wavelengths from400 to 900 nm, detectors between 800 and 900 nm would see some lightfrom wavelengths between 400 and 450 nm. This problem is normallyaddressed by placing additional filters in the system which transmitlight at a desired wavelength and absorb light at the half wavelength.Such filters may be inserted into the light path by moving mechanicalmeans or may be inserted into the dispersive and focusing system tointercept light rays reaching the longer wavelength detectors only.These filters are generally called order sorting filters. If theadditional filters are used improperly or scatter light within thedispersive and focusing system, then they become another source of straylight. If the wavelength range of the instrument is broad enough, itbecomes difficult to fabricate optical elements which perform well overthe total range of wavelengths, thus forcing the sacrificing of optimalperformance in some areas to obtain acceptable performance over thecomplete range.

SUMMARY OF THE INVENTION

It is an object of this invention to provide an optical system for amultidetector array spectrograph included as a component of an opticalsystem or instrument for the analysis of light energy as a function ofwavelength where multiple slits are used in conjunction with a singlefixed dispersive and focusing system and a collection of detectors atpositions fixed with respect to the dispersive and focusing system.

It is another object of this invention to provide a multiple entranceslit spectrograph which achieves a multiple wavelength range withoutalignment errors, stray light sources and high cost associated withsingle slit, multiple wavelength detectors which rely upon moving partsto provide multiple wavelength capabilities.

It is still another object of this invention to use a single, fixedposition grating to direct selected portions of the spectra onto asingle medium resolution detector array.

Yet another object of this invention is to provide a low cost, easilyfocused, high resolution detector array which eliminates both the costand complications of multiple grating systems.

Still yet another object of this invention is to provide a spectrographwhere the use of multiple wavelength ranges dispersed on the same set ofdetectors improves the performance of the system by allowing the use offewer detectors and smaller physical range of detector positions for thesame resolution.

Yet another object of this invention is to provide a multiple wavelengthrange spectrograph which does not require movement of the optics,sensors, or dispersive elements to achieve switching between wavelengthranges.

Still yet another object of this invention is to provide a multidetectorarray spectrophotometer where experimental results may be easilyrepeated.

Yet another object of this invention is to reduce or eliminate therequirement for order sorting filters within the dispersive and focusingsystem while allowing for a total instrument wavelength range whichexceeds a factor of two.

These and other objects and advantages are achieved by the presentinvention of an optical system for a multidetector arrayspectrophotometer which may include multiple light sources for emittinglight of selected wavelength ranges and means for selectivelytransmitting the selected wavelength ranges of light to respective slitsof a multi-slit spectrograph for multiple wavelength range detection.The spectrograph has two or more slits which direct the selectedwavelength ranges of the light spectra to fall upon a dispersive andfocusing system which collects light from each slit, disperses the lightby wavelength and refocuses the light at the positions of a single setof detectors. Such a dispersive and focusing system may include aholographic concave grating which splits the wavelength range of emittedlight falling upon the grating into its spectral components and directsthe spectra to a fixed photodiode array.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be better understood and further advantages and usesthereof are readily apparent, when considered in view of the followingdetailed description of the exemplary embodiments, taken together withthe accompanying drawings in which:

FIG. 1 is a diagrammatic view of the new optical system for amultidetector array spectrophotometer of the present invention whichillustrates the path of a first selected wavelength range of the lightspectra; and

FIG. 2 is a diagrammatic view of the optical system of FIG. 1 whichillustrates the path of a second selected wavelength range of the lightspectra.

FIG. 3 is an enlarged diagrammatic view useful to explain and define therelative positions of various elements, together with the angles betweensuch various elements, of the optical system in accordance with theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

While the description of the preferred embodiment of the inventiondisclosed herein is a specific example where sample absorptioncharacteristics are desired for two specific segments or wavelengthranges of the white light spectrum, it is entirely contemplated by theinventors that numerous modifications of the invention to allow thestudy of all segments of the light spectra or any number of selectedwavelength ranges are possible by modifications of the types of lightsources, filters or location and/or number of slits without departingsubstantially from the teachings of the invention as set forth below. Inparticular, it is contemplated that this invention shall be applied toinstruments which evaluate energy distribution as a function ofwavelength of light emitted from samples or sources and instrumentswhich evaluate the energy distribution of light as a function ofwavelength from samples in conjunction with other parameters of thesample such as position on the sample, temperature of the sample, timeor other sample conditions.

Turning first to FIG. 1, the present invention of a multi-wavelengthrange spectrophotometer includes means for generating plural wavelengthranges of light such as a light source 10 for emitting a first selectedrange of wavelengths and light source 12 for emitting a second selectedrange of wavelengths. For example, light source 10 may be a deuteriumsource for emitting light in the lower UV wavelength range of 190-545 nmand light source 12 may be a tungsten source for emitting light in thehigher VIS (or visible) wavelength range of 545-900 nm. Light source 10directs along line "a" toward mirror 14. Mirror 14 reflects the 190-545nm wavelength light along line "b" towards focusing mirror 18. Theorientation of focusing 15 mirror 18 is controlled by a motor (notshown) such as a low power dc motor of conventional design inconjunction with the appropriate linkage (also not shown), which permitsthe rotation of focusing mirror 18 between two mechanical stops whichdetermine first and second positions for mirror 18. In such a manner,mirror 18 is capable of selectively reflecting either light generated bysource 10 or source 12. For example, in FIG. 1, focusing mirror 18 isshown in a first position which would direct light from source 10 andmirror 14 along path "e" while light from source 12 and mirror 16 wouldbe directed along a path somewhere between mirror 14 and source 10. Insuch a manner, focusing mirror 18 is positioned to select light fromsource 10. By rotating mirror 18 approximately 15-20 degreescounterclockwise, mirror 18 will move to a second position as shown inFIG. 2 which will direct light from source 12 and mirror 16 along path"f" (see FIG. 2) and which will direct light from source 10 and mirror14 somewhere between mirror 16 and source 12.

The operation of the spectrophotometer of the present invention whenlight source 10 is selected for reflection by the proper orientation offocusing mirror 18 shall now be described. Light originating from source10 is reflected by focusing mirror 18 along line "e" where it passesthrough a rotatable filter wheel 20. Filter wheel 20 is rotatable by amotor (not shown) of conventional design such as a 1.8 degree steppermotor such that any one of a plurality of filters 21 may be selectivelyoriented with a high degree of precision so that light reflected byfocusing mirror 18 will pass through the selected filter 21. In such amanner, filter 21 of filter wheel 20 may filter light wavelengths notpart of the wavelength range under investigation from passing throughsample compartment 22. Additional positions on the filter wheel may beused to modify the energy as a function of wavelength as emitted by thelamp to improve overall performance or to achieve specific functionssuch as wavelength alignment or protection of samples sensitive tospecific wavelength ranges. In such a manner, filter wheel 20 provideslight modifying means capable of optimizing the energy of light reachingthe sample as a function of wavelength to match the wavelength range tobe analyzed or to allow analysis of light from the sample as a functionof the wavelength of light reaching the sample. Also, filter wheel 20should include at least one filter position 21a where, if filter wheel20 is rotated to that position, all light reflected by focusing mirror18 will be blocked. Such a configuration is desirable when a darkreference is used or when no absorption measurements are being taken andit is desirable to keep light off the sample under investigation.

Light emitted by source 10 is directed by focusing mirror 18 along line"e" such that the light passes through filter 21 and sample compartment22. Preferably, sample compartment 22, which contains a sample orcomponent of which analysis is desired, is located such that the lightreflected by focusing mirror 18 is focused at the location of samplecompartment 22. In a typical experiment, the sample contained in samplecompartment 22 may absorb at least part of the spectrum of light passingthrough the sample. It is the analysis of light which passes through thesample unabsorbed which is utilized for any one of the aforementionedmethods of analyzing the sample.

Light passing through sample compartment 22 travels along line "g'"towards lens 24. Lens 24 directs light passing through samplecompartment 22 on parallel paths along line "g'" towards mirror 29.Alternately, lens 24 may refocus light such that the beam emerging fromit and reflected by mirrors 29 and 30 will be focused on slit 40, or ifmirror 29 is not in this beam, be reflected by mirrors 31 and 32 and befocused on slit 42.

Spectrophotometer 25 comprises a first spectrophotometer compartment 26and a second spectrophotometer compartment 27. Compartment 27 is formedby two castings attached together by conventional means. Casting 27a ismounted to casting 27b and the two are secured in a precise locationwith respect to each other by pins 27c. In such a manner, slit 40, slit42 and detector means 46 such as a photodiode array 46, all of which areincluded as parts of compartment 27a and grating 44, which is mounted tocasting 27b may be precisely located with respect to each other.

In such a manner, a multi-slit spectrograph wherein a single dispersive,focusing and detecting system such as a single grating and detectorarray may be used to analyze various selected wavelength ranges isprovided. The division of the total instrument wavelength range into twoor more ranges in a system which has no moving parts within thedispersive and focusing system may be sealed against air leaks andpurged with a known gas or exposed to a drying agent allows for thedevelopment of high reliability measuring instruments which will operateover a wider range of environmental conditions than prior instruments.Further, the division of the total instrument range into two or moreranges by the use of multiple entrance slits allows the slits to befabricated as an opaque layer on optical material which may be chosen toprovide some or all of the order sorting filter required for the rangeof wavelengths to be analyzed when light passes through the particularentrance slit. Further, the division of the total instrument wavelengthrange into two or more ranges by the use of multiple entrance slits withparticular optical elements associated with each slit allows theparticular optical elements to be designed, constructed, and treated tooptimize performance in the wavelength range particular to thewavelength range with which the optical elements are associated.

Again referring to FIG. 1, light re-directed either along a parallelpath by lens 24 and travelling along line "g'" enters firstspectrophotometer compartment 26 through an opening 28 sized to permitthe entire beam of light emerging from lens 24 to enter firstcompartment 26. Generally speaking, first compartment 26 is intended toplace a tangential image of the light entering through opening 28 ontoone of two corresponding slits 40 or 42 as to be more fully describedlater. As such, the focal lengths, orientations and spacing of mirrors29, 30, 31 and 32, which are mounted within compartment 26, are selectedto achieve such a goal.

As mentioned, first spectrophotometer compartment 26 includes mirrors29, 30, 31 and 32 for redirecting light entering compartment 26. Mirrors30, 31 and 32 are fixedly mounted within compartment 26 usingconventional means not shown in the drawings. Mirror 29 is mounted onconventional mounting means 33 which is attached via an arm 36 to astepper motor 38 of conventional design capable of moving mounting means33 via arm 36. Stepper motor 38 is thereby enabled to move mirror 29into and out of the path of light entering compartment 26. For example,motor 38 may be a 1.8 degree stepper motor of conventional designcapable of precisely repositioning mirror 29 out of the light pathwithout any gear reductions.

When placement of mirror 29 in the path of light entering compartment 26is desired, mirror 29 is placed in a first position in the plane ofreflection such that all light entering compartment 26 will reflect offmirror 29 towards mirror 30. Fixed mirror 30 is positioned withincompartment 26 such that all light reflected by mirror 29 will bereflected off mirror 30 as well. Fixed mirror 31 is positioned withincompartment 26 such that when mirror 29 is moved to a second positionout of the plane of reflection for the path of light enteringcompartment 28, all light entering compartment 26 will be reflected offmirror 31 towards mirror 32. Fixed mirror 32 is positioned withincompartment 26 such that all light reflected by mirror 31 will bereflected off mirror 32 as well. In such a manner, positionable mirror29 and fixed mirrors 30, 31 and 32 are configured to permit alternatereflection of entering light energy by mirrors 29 and 30 or by mirrors31 and 32. By rotating mirror 29 in its plane of reflection into or outof the path of the light from lens 24, the position of mirror 29 selectsslit 40 or 42. Because the plane of rotation is the plane of reflection,precise positioning of the mirror is not required and stepping errors donot affect the angles of rays reflecting from the mirror.

It is further contemplated that in the embodiment of the invention wheremultiple slits are utilized in conjunction with multiple generatedwavelength ranges of light, mirror 29 may be replaced with a pluralityof mirrors in multiple parallel planes. In this embodiment, theplurality of mirrors may intercept and reflect light enteringcompartment 28 such that the entering light corresponding to a multiplenumber of wavelength ranges may be selectively directed to acorresponding one of an equal number of entry ports or slits provided aspart of the spectrograph of the present invention.

As previously noted, the operation of the optical system for thespectrophotometer of the present invention is presently being describedfor the case where light in the UV wavelength range of 190-545 nmemitted from light source 10 has been selected to pass through samplecompartment 22 and into compartment 26. For this particular example,when light emitted by source 10 is selected to enter compartment 26,mirror 29 is positioned in the plane of reflection of the path of lightentering compartment 26 and will reflect the incoming light energy alongpath "j" towards fixed mirror 30. Fixed mirror 30 reflects the lightenergy again such that the light will travel along path "1" and towardsslit 40. Since mirrors 29 and 30 are associated with slit 40 and areintended to pass UV light, mirrors 29 and 30 may be optimized for UVimaging and energy transfer while suppressing non UV energy performanceby proper design and coating.

The interface between compartments 26 and 27 includes first entry portor slit 40 and second entry port or slit 42. Preferably, slits 40 and 42are spaced apart such that light reflected off mirror 30 converges atslit 40 and light reflected off mirror 32 converges at slit 42. In thespecific embodiment disclosed, the desired result is achieved by spacingslits 40 and 42 slightly apart and placing mirror 30 slightly furtheraway from its corresponding slit 40. In such a manner, when light passesthrough slit 42, the second selected wavelength range (here, the higherVIS light wavelengths of the spectra) under investigation will bedetected by array detector 46, and when light passes through slit 40,the first selected wavelength range (here, the lower wavelengths UVspectra) under investigation will fall upon the same array detector. Insuch a manner, a multi-slit spectrograph wherein a single dispersive,focusing and detecting system such as a single grating and detectorarray may be used to analyze various selected wavelength ranges.

To achieve such an effect, slits 40 and 42 are different both in anglewith respect to the grating 44 and detector 46 and in distance fromgrating 44 in order to optimize focus for the selected wavelengthsranges of the spectrum under investigation. It should be clearly noted,however, that the specific orientation of slits 40 and 42 with respectto each other may be varied depending on the selected wavelength rangesor the orientation of reflecting mirrors 29, 30, 31 and 32 and thepresent invention should not be restricted to any specific slitorientation.

Light of the first selected wavelength which passes through slit 40,enters compartment 27 and travels along path "n" where it is dispersedby wavelength and re-focused by holographic grating 44, which is fixedto second spectrophotometer compartment 27 by conventional means, to aphotodiode array 46 such as a linear 1024 element photodiode array. Thephotodiode array may be any one of numerous types available in themarketplace. The desired dispersion in the UV range determines the anglefrom the center of the grating to UV slit 40 and the distance betweengrating 44 and slit 40 is chosen for best focus on photodiode array 46.UV slit 40 is fabricated by photoetching chrome on a piece of quartz.Since the wavelength range falling on the detector when light passesthrough slit 40 is intended to be in the UV range, the physical materialof this slit may be selected to transmit UV wavelengths whilesuppressing wavelengths not detected when this slit is in use. Thisselection of slit material and fabrication further improves systemperformance by decreasing error due to light at incorrect wavelengths.

The orientation of focusing mirror 18 is controlled by a motor (notshown) which, in conjunction with the appropriate linkage (also notshown), may be rotated such that the mirror 18 is capable of selectivelyreflecting either light generated by source 10 or source 12.

Turning next to FIG. 2, the operation of the spectrophotometer of thepresent invention when light source 12 is selected for generating asecond selected wavelength range (here, a tungsten source of wavelengthrange of 545-900 nm) when sample absorption studies for the secondwavelength range is desired. Light emitted by source 12 is directedalong line "c" towards reflecting mirror 16. Mirror 16 reflects the545-900 nm wavelength light along line "d" towards focusing mirror 18.Focusing mirror 18, which has been repositioned by the aforementionedmotor such that mirror 18 will reflect light transmitted by source 12only, reflects light transmitted by mirror 16 along line "f" where itpasses through rotatable filter wheel 20. Prior to repositioningfocusing mirror 18 to transmit light from source 12, filter wheel 20should be rotated to a next position so that a next filter 21 bettercapable of filtering stray light not part of the second wavelength rangeunder investigation will be positioned in the path of light beingemitted by source 12.

Light emitted by source 12 is directed by focusing mirror 18 along line"f" such that the light passes through next filter 21 and samplecompartment 22 containing the sample previously analyzed for thewavelength range emitted by source 10. Again, the sample contained insample compartment 22 may absorb a part of the spectrum of the lightpassing through the sample. Light passing through sample compartment 22travels along line "h" towards lens 24.

Lens 24 re-directs light passing through sample compartment 22 along aparallel path along line "i" towards mirror 31. Alternately lines 24 maybe a focusing lens which would instead re-focus light passing throughsample compartment 22 into a converging beam of light such that lightemerging from lens 24 will be focused at a point withinspectrophotometer 26 and before mirror 31. Light re-directed alongparallel paths by lens 24 and travelling along line "i" enters firstspectrophotometer compartment 26 through an opening 28 sized to permitthe entire beam of light emerging from lens 24 to enter firstcompartment 26.

So that the second wavelength range, i.e. light emitted by source 12,will pass through slit 42 instead of slit 40, mirror 29 is moved to thepreviously disclosed second position located out of the path of lightentering first compartment 26 through opening 27 prior to the emissionof light by source 12. In such a manner, all light entering compartment26 which originated from source 12 will be reflected off mirror 31towards mirror 32.

Mirror 31 will thereby reflect light entering compartment 26 along path"k" towards fixed mirror 32. Fixed mirror 32 reflects the light energyagain such that the light will travel along path "m" and towards slit42. Since mirrors 31 and 32 are associated with slit 42 and are intendedto pass visible light, mirrors 31 and 32 may be optimized for visiblelight imaging and energy transfer while suppressing energy transfer inother wavelength ranges by proper design and coating. Light passingthrough slit 42 enters spectrophotometer compartment 27, travels longpath "o" and is dispersed by wavelength and refocused by holographicgrating 44 onto photodiode array 46. Here, the desired dispersion in thevisible range determines the angle from the center of the grating tovisible slit 42 and the distance between grating 44 and slit 42 ischosen for best focus on detector array 46. Since the wavelengths oflight reaching the detector when light passes through slit 42 areintended to be in the visible range, slit 42 may be constructed byphotoetching chrome on a piece of optical material chosen to suppresswavelengths outside this range, thus improving system performance bydecreasing error due to light at incorrect wavelengths reaching thedetector.

The preferred embodiment of the relative positions of port 40, port 42,grating 44, and detector 46 may be further described by defining theangles between the grating and the ports and between the grating and thedetector. This is shown in FIG. 3.

It is the nature of a grating to disperse light by wavelength in apreferred plane. FIGS. 1, 2, and 3 are drawn such that this plane is theplane of the paper. This discussion refers to angles in this preferredplane or projected onto this plane. To make this description clear, wefirst define a line normal to the surface of the grating at the centerof the grating to be the grating normal. For purposes of thisdiscussion, this is a convention used to simplify the description of therelative positions of these components. Then, using the chosen gratingnormal, we may define angles from the grating normal as rotation aboutthe point at the intersection of the grating normal and the gratingsurface. That is, the concave grating 44 has a preferred plane todisperse light and the concave grating 44 has a normal lying in thepreferred plane relative to which angles may be defined as rotationsaround a rotation point located at the intersection of the gratingnormal and the surface of the grating. Using the convention thatclockwise rotation corresponds to positive angles, the preferredembodiment would be further described as follows:

The angle from grating normal to the center of the port 40, labeled A40on FIG. 3, is in the range between -5.2 and -5.4 degrees and optimal at-5.336 degrees.

The angle from grating normal to the center of the port 42, labeled A42on FIG. 3, is in the range between -0.3 and -0.5 degrees and optimal at-0.421 degrees.

The angle from grating normal to closest sensor position or element ondetector 46, labeled A46C on FIG. 3, is more positive than 8.3 degreesand optimal for the 190 and 545 nm wavelengths at 8.437 degrees.

The angle from grating normal to farthest sensor position or element ondetector 46, labeled A46F on FIG. 3, is more negative than 14.4 degrees,i.e. is rotated counterclockwise from 14.4 degrees, and optimal for 545and 900 nm wavelengths at 14.304 degrees.

It should be clear that by choosing the convention that counterclockwiserotation corresponds to positive angles, the same embodiment would bedescribed with all signs reversed.

It should also be clear that other variations in the convention fordefining the grating normal are possible which lead to the sameembodiment. In particular, the grating normal could be defined as a linenormal to the grating surface from a point chosen on the grating surfacesuch that the dispersion of light by the grating is symmetrical aboutthe normal so defined.

It should also be clear that other port positions between -0.3 degreesfrom the grating normal and -5.4 degrees from the grating normal can bechosen to replace either port 40 or port 42 or as additional portpositions.

In accordance with the invention, the positions or locations of thelight entry ports relative to the concave grating 44 are chosen so as toeffect optimum focus of the light entering at the ports at the detectormeans 46. As for example, with center of the first port 40 being chosento be at a specific angle chosen to be between -0.3 and -5.4 degreesfrom the grating normal, the location of port 40 along a line defined bythe chosen angle and the rotation point (located at the intersection ofthe grating normal and the surface of the grating) is selected so as toobtain optimum focus of light entering the port 40 at the detector means46 by the grating 44. Likewise for port 42, with the center of thesecond port 42 being chosen to be at a specific angle chosen to bebetween -0.3 and -5.4 degrees from the grating normal, the location ofport 42 along a line defined by the chosen angle and the rotation pointis selected so as to obtain optimum focus of light entering the port 42at the detector means 46 by the grating 44. Of course, the same methodwould be used to locate any additional light entry ports so as to effectoptimum focus of light by the grating 44 at the detector means 46.

Thus, there has been described herein, apparatus for a new opticalsystem for a multidetector array spectrograph which provides amulti-slit configuration which permits the use of a single dispersingand focusing system such as fixed grating and a single photodiode lineararray detector for recording multiple wavelength ranges such as a highlight wavelength range and a low light wavelength range which are bothpassed through a sample. However, those skilled in the art willrecognize that many modifications and variations may be made in thetechniques described herein without departing substantially from theconcept of the present invention. Accordingly, it should be clearlyunderstood the form of the invention described herein is exemplary onlyand is not intended as a limitation upon the scope of the presentinvention.

I claim:
 1. An apparatus for the measurement of light intensity bywavelength comprising:a concave grating for the dispersion and focusingof light by wavelength; said concave grating having a preferred plane todisperse light and having a normal lying in said preferred planerelative to which angles may be defined as rotations around a rotationpoint located at the intersection of said normal and the surface of saidgrating, detector means optically coupled to said concave grating forthe conversion of light energy to electrical energy indicative of saidlight intensity; said detector means comprising a plurality of lightsensitive elements, said detector means being positioned such that saidlight sensitive elements are present at or between angles of 8.3 and14.4 degrees from the grating normal, a first port for entrance of lightto the apparatus; the center of said first port being at a specificfirst angle chosen to be between -0.3 and -5.4 degrees from the gratingnormal, the location of said first port along a line defined by saidchosen first angle and said rotation point selected so as to obtainoptimum focus of light entering at said first port at said detectormeans by said grating, and a second port for entrance of light to theapparatus; the center of said second port being at a specific secondangle chosen to be between -0.3 and -5.4 degrees from the gratingnormal, the location of said second port along a line defined by saidchosen second angle and said rotation point selected so as to obtainoptimum focus of light entering at said second port at said detectormeans by said grating.
 2. Apparatus according to claim 1 whereinsaidfirst chosen angle for said first port is between -5.2 and -5.4 degreesfrom the grating normal, and said second chosen angle for said secondport is between -0.3 and -0.5 degrees from the grating normal. 3.Apparatus according to claim 2 whereinsaid first chosen angle is -5.336degrees from the grating normal, and said second chosen angle is -0.421degrees from the grating normal.
 4. Apparatus according to claim 1further comprising at least one additional port for entrance of light tothe apparatus; the center of said one additional port being at aspecific additional angle chosen to be between -0.3 and -0.54 degreesfrom the grating normal, the location of said additional port along aline defined by said chosen additional angle and said rotation pointselected so as to obtain optimum focus of light entering at saidadditional port at said detector means by said grating.